Flexible sheet-like magnetic discs having a nominal standardized size of about 3.5 inches are widely used in a variety of data storage/retrieval systems. For example, cassettes which include flexible magnetic discs are used in conjunction with personal computers so as to load data and/or programs into the central processing unit of the computer, as well as to store data in an off-site location in a more convenient manner.
When placed into service in a magnetic recording/reproducing apparatus, the magnetic disc (which is accommodated for rotational movement within the interior of the cassette case) is caused to spin relative to a magnetic read/write head by means of a motor-driven spindle coacting with an aperture in the central hub of the magnetic disc. The spindle, moreover, serves to center the magnetic disc relative to the magnetic read/write head so that accurate placement and retrieval of data onto and from the disc will ensue.
The cassette case for flexible magnetic discs having a standardized size of about 3.5 inches (i.e., so-called "microdiscs") is relatively thin (i.e., on the order of about 0.14 inch (3.4 mm)). The motor-driven spindle thus enters the cassette case through an opening defined in the lower cassette half and then extends through the aperture in central hub of the magnetic disc such that its terminal end contacts an interior surface portion of the upper cassette half. Contact between the terminal end of the motor-driven spindle and the interior surface portion of the upper cassette half sometimes also causes the cassette case to bow or flex in its central region so as to create more clearance space between the case and the magnetic disc and thereby facilitate spinning of the latter relative to the former.
Conventional cassette cases are usually formed of a plastics material (e.g., acrylonitrile-butadiene-styrene (ABS) terpolymer) which has the necessary mechanical properties needed to protect the magnetic disc. However, cassette cases formed of such plastics materials do not have the necessary anti-friction (i.e., low coefficient of friction against steel) properties needed to serve as a bearing surface for the motor-driven spindle during use. As a result, conventional flexible magnetic disc cassette cases include a separate wear button formed of a different plastics material (e.g., a blend of high density polyethylene and a minor amount (e.g., between 10-20 wt. %, and typically around 2 wt. %) of polytetrafluoroethylene) having a lower coefficient of friction against steel as compared to the plastics material from which the cassette case is formed.
The separate wear button associated with conventional flexible magnetic disc cassettes is typically fixed to the interior portion of the upper cassette half via adhesive bonding or like techniques. As a result, there is necessarily increased handling requirements (increased labor) needed to produce conventional flexible magnetic disc cassettes. Therefore, although the provision of a separate wear button with conventional flexible magnetic disc cassettes does provide a relatively low friction surface against which the motor-driven spindle bears when the flexible magnetic disc cassette is placed into service, the manufacturing costs are necessarily increased due to increased labor/handling which ensues during production. It is towards reducing (or eliminating entirely) these added costs associated with separate wear buttons that the present invention is directed.
According to the present invention, at least the upper half of the cassette case is formed entirely of a moldable plastics material which not only meets the mechanical properties demanded for flexible cassette cases generally, but also exhibits the necessary anti-friction properties needed in order to provide a low friction surface against which the motor-driven spindle associated with a magnetic recording/reproducing apparatus can bear during use. As a result, a wear button may be provided as an integrally molded (one-piece) structure associated with the upper half of the cassette case on its interior surface in opposition to the motor-driven spindle.
The plastics material from which at least the upper half of the cassette case (and more preferably both the upper and lower halves of the cassette case) is formed according to the present invention is an oxymethylene polymer which exhibits the necessary mechanical properties (e.g., tensile, compressive, and flexural strengths) and anti-frictional property against steel. Most preferably, the integrally molded wear button according to the present invention will present a smooth convexly arcuate surface (which most prefereably is a spherical surface segment) to the terminal end of the motor-driven spindle so as to minimize the contact area therebetween, and thus further reduce frictional wear when in use.
Further aspects and advantages of the present invention will become more clear after careful consideration is given to the following detailed discussion thereof.